ABSTRACT Combining local radiation therapy and systemic immunotherapy to produce an abscopal tumor response at a distant unirradiated site has been the ?holy grail? of radioimmunotherapy. Preclinical data has shown that radiation and systemic immunotherapy can be combined to provide an enhanced response, however translation into the clinical setting has been much more difficult. These mixed results may partly be explained by immunosuppressive effects of high tumor burden or distant disease as well as the fact that many cancers are poorly immunogenic or ?cold? with low response rates to immunotherapeutic treatments. Therefore, the aims of this proposal look to enhance the efficacy of immunotherapy treatments in preclinical models of metastatic immunologically ?cold? tumors that don?t respond to traditional systemic immunotherapy such as immune checkpoint inhibitors (ICI, e.g. anti-CTLA4, anti-PD1) and most closely resembles our current target patient population today in whom we are trying to improve outcomes. Therefore, we developed a metastatic model of immunologically ?cold? melanoma with a large established primary tumor, bulky secondary disease sites, and distant disease sites that resembles the type of disease burden we need to treat in our patients in the clinic. Our preliminary data shows that to optimize cure rates in this model we require a three-prong approach. First, we utilize a strong local in situ vaccine regimen (ISV) consisting of external beam radiation therapy (12 Gy x 1) and intratumoral injection of a tumor specific antibody and IL2. Next to prime distant disease for systemic immunotherapy treatment we utilize a novel molecular targeted radiotherapy (MTRT) agent, NM600, which is a diapeutic alkylphosphocholine molecule that has been chelated to 86/90Y. These MTRT agents have previously been shown to have selective tumor uptake in virtually all mammalian tumor cells tested (including > 70 tumor lines and in patients across various clinical trials). Lastly, we will utilize systemic ICI to continue a robust adaptive immune activation response and suppress escape mechanisms. Our preliminary data shows that this combination treatment is effective in curing local, distant, and metastatic disease in an immunologically ?cold? melanoma model that doesn?t respond ICI alone. For the aims of this study we will: 1) expand on our preliminary data showing uptake of MTRT to sites of distant disease and calculate tumor dosimetry 2) demonstrate the ability of MTRT to modulate a tumor microenvironment at both distant and local sites to enhance the efficacy of immune response, and 3) test the efficacy of this treatment approach in a variety ?cold? tumor models such as melanoma (B78), neuroblastoma (NXS2), head and neck cancer (MOC2), breast cancer (4T1), and a spontaneously arising transgenic melanoma model. The insights and knowledge gained by this proposal should allow us to provide valuable justification to translate this treatment to clinical testing in patients and potentially improve outcomes in patients with any type of metastatic cancer.